Research is proposed to 1) determine what mechanisms of peptide influence on the E 1/2 of 4Fe-4S clusters functions in iron-sulfur proteins, and 2) determine if cysteinyl sulfur atoms exposed to the solvent in C. pastuerianum ferredoxin provide the pathway for electron transfer. NMR experiments are described to determine if the reduction potential is altered by peptide constraint of the iron-sulfur cluster to specific geometries. These studies involve primarily A. vinelandii ferredoxin I. As a partial test of electrostatic effects of the peptide, NMR spectra of R. gelatinosa and Chromatium HiPIPs will be recorded. Finally the origin of the shift in E 1/2 in C. acidi-urici ferredoxin will be tested by replacement of the single amino acid Tyr 2 with either His or Trp and observation using NMR and EPR. The possibility of cysteinyl sulfur atoms providing the pathway for electron transfer in C. pasteurianum ferredoxin will be tested by use of (terpy)PtCl plus and Ru(NH3)5H2O 2 plus to modify the exposed cysteinyl sulfur atoms. By a series of biological assays and kinetic studies it will be possible to determine if cysteinyl sulfur atoms provide the pathway for electron transfer.